1. Field of the Invention
This invention relates to optical fibers. More specifically, this invention relates to optical fibers employed in fiber optic sensors and interferometers.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
Considerable progress has been made recently in the development of fiber optic sensors. This is due in part to the advantages afforded by fiber optic sensors over conventional sensors. Chief among these are increased sensitivity, faster response, geometric versatility, substantial immunity to electromagnetic interference, large bandwidth for high speed information transmission, and the capability to measure a variety of physical phenomenon (i.e. temperature, pressure, strain, rotation, and acoustic and magnetic fields). In addition, the dielectric construction of fiber optic sensors allows use in high voltage, high temperature, or corrosive environments. Finally, the electronic and opto-electronic components used to monitor the optical energy propagating through the fiber may be remotely located with respect to the sensing environment.
In conventional fiber optic sensors light guided within an optical fiber is modified in reaction to various external physical, chemical or similar influences. In operation, light from a source having relatively stable optical properties is typically coupled into the fiber of the conventional sensor. The light is then directed by the fiber to a region in which a measurement is to take place. In extrinsic sensors the guided light then may exit the fiber and interact with the substance being measured (measurand) prior to being launched into the same or a different fiber. Alternatively, in intrinsic sensors the light remains within the fiber throughout the measurement region.
In extrinsic sensors reflection losses are incurred as the guided light leaves the optical fiber and enters the measurand. In particular, these reflection losses are proportional to the difference between the indices of refraction of the fiber and the measurand. At present, the minimum index of refraction (at visible wavelengths) of optical fibers is believed to be approximately 1.4. Consequently, reflection losses are generally unavoidable when extrinsic sensors are employed to analyze material having an index of refraction (n) less than 1.4. For example, in the visible region water exhibits an index of refraction of approximately 1.33.
In another type of optical fiber sensor, generally known as an evanescent field sensor, the light guided by the fiber partially couples to the measurand via an evanescent (i.e. exponentially decaying) field which surrounds the fiber. In evanescent mode sensors, a cladding sheath surrounding the optical fiber core is made sufficiently thin such that an evanescent mode is supported by the fiber. In an evanescent mode, a portion of the optical energy carried by the fiber propagates along the length of the fiber within a region of space immediately surrounding the cladding. The measurand surrounding the fiber may either absorb or change the properties of the evanescent field, thus enabling a measurement to be performed. Employment of evanescent mode over extrinsic mode fiber sensors may be preferred in applications requiring direct interaction between the light beam and the measurand since evanescent mode requires no relaunching of the beam.
A distinction may also be made between interferometric and intensity-modulated fiber optic sensors. It is generally recognized that interferometric sensors provide improved sensitivity, wider dynamic range, and better accuracy than intensity modulated sensors. In the latter, intensity fluctuations of light propagating through a fiber disposed in the measurand are directly monitored in order to generate a detection signal. The sensor is designed such that the detection signal is indicative of changes occurring in a particular physical characteristic of the measurand.
In contrast, within interferometric sensors, light traversing two or more optical paths is coherently mixed such that an interference pattern is formed. The phase difference between light from the constituent optical paths may then be discerned by analyzing the interference pattern. In this manner interferometric sensors are designed such that a change in a physical characteristic of the measurand affects the measured phase difference, thus allowing a detection signal to be synthesized in response thereto.
In the field of interferometric fiber optic sensors, those utilizing multiple-beam interferometers are typically more accurate than those employing dual-beam devices such as the Michelson interferometer. Among multiple-beam interferometers, those identified as being of the Fabry-Perot variety are widely employed. Specifically, Fabry-Perot interferometric sensors operate to yield well-defined interference patterns by combining multiple optical reflections from within a single fiber. The fiber defines a resonant cavity which is typically bounded by a pair of partially reflective mirrors external thereto. Alternatively, an air gap may be introduced between a pair of fiber segments to serve as a surrogate for a physical partially reflective mirror. However, this latter approach results in a high sensitivity to physical perturbation and limits the maximum reflectance which may be effected.
In an attempt to overcome such difficulties a technique for developing optical fiber segments having internal partially reflective structures has recently been proposed. Specifically, in "Fiber Optic Sensor Research at Texas A&M University"; by H. F. Taylor and C. E. Lee; Proceedings of The International Society for Optical Engineering, vol. 1170 (Fiber Optic Smart Structures and Skins II), p. 113 (1989), a technique for fabricating internal dielectric mirrors in continuous lengths of silica fiber is disclosed. The approach involves slicing a silica fiber and coating one of the exposed ends with a thin film of titanium oxide TiO.sub.2. The coated end is then joined to an uncoated end of a similarly spliced fiber using an electric fusion splicer. In this manner, a continuous length of fiber which contains multiple internal mirrors of desired reflectance may be produced.
Unfortunately, fabrication of the optical fiber suggested by Taylor will generally need to occur within a relatively particle-free environment in order to prevent contamination of the exposed internal fiber surfaces. Moreover, the construction process is further complicated by the requirement that the component fiber segments be precisely aligned during fusion splicing.
Accordingly, a need in the art exists for an improved technique for creating internal partial mirrors in optical fibers.